Antimicrobial polypeptides (APs) are key components of the primary defense mechanisms of mucus membranes, providing first-line inactivation of bacteria, fungi and viruses on the vast epithelial surfaces that line the respiratory, gastrointestinal and urogenital tracts. The potent antimicrobial activity of these cationic polypeptides, which is often synergistic and can be additive, provides the highly effective mechanism of action of the innate immune system. Recently, it has begun to be appreciated that the commensal microorganisms living on these mucosal surfaces stimulate epithelial cells to produce these effectors of innate immunity and thus contribute to host maintenance of defensive mucosal barriers and homeostasis. This latter point is particularly relevant to otitis media (OM) because OM is not caused by highly virulent microorganisms; OM is instead caused by a subset of the commensal bacteria that typically comprise the normal flora of the pediatric nasopharynx (NP). However, when host airway defenses are compromised, these bacteria can behave as opportunistic pathogens and gain access to the now poorly defended middle ear. It is well known that colonization of the NP is thus the very first step in the disease course of OM. Immediately preceding invasion of the middle ear, the bacterial load in the nasopharynx increases significantly. Moreover, otitis prone children are more heavily colonized by the bacterial species that are predominant in acute and chronic OM compared to their non-otitis prone counterparts, thus reinforcing the notion that maintaining a relatively low bacterial load in the NP is important to the health of the middle ear, particularly during times of viral compromise. It is highly likely that maintenance of the bacterial load in a child's NP at a noninfectious "colonizing" level is regulated on a day-to-day basis primarily by the innate immune system, including action of the APs and natural antibodies present in airway secretions, and not by effectors of acquired immunity. The relative effectiveness of the innate immune system in a given individual would then influence otitis proneness as well. Conversely, dysregulation of innate defense mechanisms (i.e. due to concurrent upper respiratory tract (URT) viral infection or genetic predisposition to an inadequate or hypo-responsive innate immune system) may allow an increase in bacterial load to a more 'infectious-prone' level. It is thereby important to define and study the innate immune system of the uppermost airway, which includes the nasopharynx, Eustachian tube and middle ear. Toward this goal, in the present application we propose to characterize and study the APs produced in the uppermost airway. Specifically, we wish to investigate their role in bacterial colonization of the nasopharynx and determine if their expression is up- or down-regulated in response to the predominant viral and bacterial pathogens of OM. Moreover, we propose to determine their relative level of expression during induction and resolution of experimental OM using well-established chinchilla models. Finally, we will deliver isolated recombinant or synthetic chinchilla APs (including the recently identified chinchilla beta defensin-1) to the uppermost airway of the chinchilla host in an attempt to inhibit the development of OM that occurs in this model.